We have investigated the photoionization of ${\mathrm{Ne}}^{+}$ in the combined field of a short infrared laser pulse and a delayed ultrashort pulse of the infrared laser's 23rd harmonic. We observe an ionization yield compatible with a picture in which one electron gets excited into Rydberg states by the harmonic laser field and is subsequently removed by the infrared laser field. Modulations are seen in the ionization yield as a function of time delay. These modulations originate from the trapping of population in low members of the Rydberg series with different states being populated at different ranges of delay times. The calculations further demonstrate that single-threshold calculations cannot reproduce the ${\mathrm{Ne}}^{+}$ photoionization yields obtained in multithreshold calculations.

Theoretical analysis of the excitation of low-$Z$ hydrogenlike atoms by incident twisted light is presented. Emphasis is placed on the cross sections that describe transitions between particular atomic substates. Simple expressions for these partial cross sections are derived using the nonrelativistic first-order perturbation theory and the momentum representation of the photon wave functions. Based on the developed approach, calculations have been performed for the $1s\to 2p$ and $2p\to 3d$ transitions induced in the course of interaction of twisted (Bessel) light with a macroscopic hydrogen target. Results of these calculations, supported by an analytical analysis, clearly indicate that the sublevel population of residual atoms following absorption of twisted photons differs much from what is expected for the standard plane-wave case; this effect can be easily observed experimentally by measuring the linear polarization of the subsequent fluorescent emission.

Spin-superflow turbulence (SST) in spin-1 ferromagnetic spinor Bose-Einstein condensates is theoretically and numerically studied by using the spin-1 spinor Gross-Pitaevskii (GP) equations. SST is turbulence in which the disturbed spin and superfluid velocity fields are coupled. Applying the Kolmogorov-type dimensional scaling analysis to the hydrodynamic equations of spin and velocity fields, we theoretically find that the $-5/3$ and $-7/3$ power laws can appear in spectra of the superflow kinetic and the spin-dependent interaction energies, respectively. Our numerical calculation of the GP equations with a phenomenological small-scale energy dissipation confirms SST with the coexistence of disturbed spin and superfluid velocity field with two power laws.

We studied the appearance of Mott insulator domains of hard sphere bosons on quasi-one-dimensional optical lattices when a harmonic trap was superimposed along the main axis of the system. Instead of the standard approximation represented by the Bose-Hubbard model, we described those arrangements by continuous Hamiltonians that depended on the same parameters as the experimental setups. We found that for a given trap the optical potential depth, $V_0$, needed to create a single connected Mott domain decreased with the number of atoms loaded on the lattice. If the confinement was large enough, it reached a minimum when, in the absence of any optical lattice, the atom density at the center of the trap was the equivalent of one particle per optical well. For larger densities, the creation of that single domain proceeded via an intermediate shell structure in which Mott domains alternated with superfluid ones.

We systematically investigate the topological superfluidity of a Rydberg-dressed Fermi gas, where the soft-core effective interaction is of finite radius $R_c$ due to the blockade effect. We calculate the quantum phase diagram within the mean-field approximation and find three distinct phases: polar $\left(p_z\right)$, axial $\left(p_x+i{p}_y\right)$, and axiplanar $\left(p_x+i{\beta }_p{p}_y\right)$ phases. The tricritical point is around $R_c{k}_F\sim 1$, where $k_F$ is the Fermi wave vector. We further derive the Ginzburg–Landau theory to explain the phase diagram and estimate the transition temperature to be about $0.1E_F$ in the current experimental regime of ${}^6\mathrm{Li}$. Our work paves the way for future studies on exotic topological superfluids and related quantum phase transitions.

In traditional quantum optics, where the interaction between atoms and light at optical frequencies is studied, the atoms can be approximated as pointlike when compared to the wavelength of light. So far, this relation has also been true for artificial atoms made out of superconducting circuits or quantum dots, interacting with microwave radiation. However, recent and ongoing experiments using surface acoustic waves show that a single artificial atom can be coupled to a bosonic field at several points wavelengths apart. Here, we theoretically study this type of system. We find that the multiple coupling points give rise to a frequency dependence in the coupling strength between the atom and its environment and also in the Lamb shift of the atom. The frequency dependence is given by the discrete Fourier transform of the coupling-point coordinates and can therefore be designed. We discuss a number of possible applications for this phenomenon, including tunable coupling, single-atom lasing, and other effects that can be achieved by designing the relative coupling strengths of different transitions in a multilevel atom.

We propose a protocol that enables strong coupling between a flux qubit and the quantized motion of a magnetized nanomechanical cantilever. The flux qubit is driven by microwave fields with suitable parameters to induce sidebands, which will lead to the desired coupling. We show that the nanomechanical modes can be cooled to the ground states and the single-mode squeezed vacuum states can be generated via fast dissipation of the flux qubit. In our scheme, the qubit decay plays a positive role and can help drive the system to the target states.

We demonstrate multiphoton blockades (PB) in the pulsed regime by using a Kerr-nonlinear dissipative resonator driven by a sequence of Gaussian pulses. It is shown that the results obtained for single-photon, two-photon, and three-photon blockades in the pulsed excitation regime differ considerably from analogous results obtained for the case of continuous-wave (cw) driving. We strongly demonstrate that for the case of cw pumping of the Kerr-nonlinear resonator there are fundamental limits on populations of lower photonic number states (with $n=0$, 1, 2, 3). Thus, such detailed comparison demonstrates that PB due to excitation with a suitable photon pulses is realized beyond the fundamental limits established for cw excitations. We analyze photon-number effects and investigate the phase-space properties of PB on the base of photon number populations, the second-order correlation functions, and the Wigner functions in phase space. The generation of Fock states due to PB in the pulsed regime is analyzed in details.

We present an ab initio treatment of the steady-state of lasers with injected signals that describes a regime, valid for microlasers, in which the locking transition is dominated by cross saturation and spatial hole burning. The theory goes beyond standard approaches and treats multimode lasing with injected signals and finds the possibility of partially locked states and as well as repulsion of the free-running frequencies from the injected signal. The theory agrees well with exact integration of the full wave and matter equations for the system. It can also describe accurately complex modern lasers structures and is applied to the example of deformed disk lasers. We show that in the case of a one-dimensional cavity in the locked or regenerative amplifier regime the theory reduces to an improved version of the Adler equations in the appropriate limit.

The asymmetry properties of a state relative to some symmetry group specify how and to what extent the given symmetry is broken by the state. Characterizing these is found to be surprisingly useful for addressing a very common problem: to determine what follows from a system's dynamics (possibly open) having that symmetry. We demonstrate and exploit the fact that the asymmetry properties of a state can be understood in terms of information-theoretic concepts. We show that for a pure state $\psi$ and a symmetry group $G$, they are completely specified by the characteristic function of the state, defined as ${\chi }_{\psi }\left(g\right)\equiv \langle \psi \left|U\left(g\right)\right|\psi \rangle$, where $g\in G$ and $U$ is the unitary representation of interest. Based on this observation, we study several important problems about the interconversion of pure states under symmetric dynamics, such as determining the conditions for reversible transformations, deterministic irreversible transformations, and asymptotic transformations.

We give a conceptually simple necessary condition such that a separable quantum operation can be implemented by local operations on subsystems and classical communication between parties (LOCC), a condition which follows from a novel approach to understanding LOCC. This necessary condition applies to all LOCC protocols involving any number of parties and any finite number of rounds of communication. Furthermore, it demonstrates an extremely strong difference between separable operations and LOCC, in that there exist examples of the former for which the condition is extensively violated. More precisely, the violation by separable operations of our necessary condition for LOCC grows without limit as the number of parties increases.

We present a protocol for quantum state transfer and remote state preparation across spin chains which operate in their antiferromagnetic mode. The proposed mechanism harnesses the inherent entanglement of the ground state of the strongly correlated many-body systems which naturally exists for free. The uniform Hamiltonian of the system does not need any engineering and, during the whole process, remains intact while a single-qubit measurement followed by a single-qubit rotation are employed for both encoding and inducing dynamics in the system. This, in fact, has been inspired by recent progress in observing spin waves in optical lattice experiments, in which manipulation of the Hamiltonian is hard and instead local rotations and measurements have become viable. The attainable average fidelity stays above the classical threshold for chains up to length 50 and the system shows very good robustness against various sources of imperfection.

We present a comprehensive implementation of the quantum trajectory theory for the description of the entanglement dynamics in a Markovian open quantum system made of two qubits. We introduce the average concurrence to characterize the entanglement in the system and derive a deterministic evolution equation for it that depends on the ways in which information is read from the environment. This buildt-in flexibility of the method is used to address two actual issues in quantum information: entanglement protection and entanglement estimation. We identify general physical situations in which an entanglement protection protocol based on local monitoring of the environment can be implemented. Additionally, we methodically find unravelings of the system dynamics providing analytical tight bounds for the unmonitored entanglement in the system at all times. We conclude by showing the independence of the method from the choice of entanglement measure.

Quantum random walks have received much attention for their intrinsic interest and many possible uses and have been experimentally demonstrated. In this work we look at the possibility of using a biased one-dimensional (1D) quantum walk as an element within a larger quantum device. We ask whether one can use a quantum walk to act as a router with one bias setting engineering the quantum walk to route probability flow one direction while another bias setting routes flow in the opposite direction. Appealing to electrical circuit terminology, we consider a biased quantum walk over a large spatial lattice to act as a single “lumped element” whose routing action depends on the coin bias. We discover that the lumped-element current, when summed over the quantum walk lattice, reaches a steady state and for specific initial states we derive an analytic form for this steady-state lumped-element current. We show that we can control the magnitude and the direction (routing) of the steady-state current. Curiously the control phase and steady-state total current exhibits a sinusoidal current-phase relationship indicating that the lumped element may be similar to that found in Josephson junctions. Finally we illustrate that conservative 1D Hamiltonian systems can also exhibit steady-state dynamics similar to the quantum walk.

The static and dynamic electric multipolar polarizabilities and second hyperpolarizabilities of the ${{\mathrm{H}}_2}^{+}$, ${{\mathrm{D}}_2}^{+}$, and ${\mathrm{HD}}^{+}$ molecular ions in the ground and first excited states are calculated nonrelativistically using explicitly correlated Hylleraas basis sets. The calculations are fully nonadiabatic; the Born-Oppenheimer approximation is not used. Comparisons are made with published theoretical and experimental results, where available. In our approach, no derivatives of energy functions nor derivatives of response functions are needed. In particular, we make contact with earlier calculations in the Born-Oppenheimer calculation where polarizabilities were decomposed into electronic, vibrational, and rotational contributions and where hyperpolarizabilities were determined from derivatives of energy functions. We find that the static hyperpolarizability for the ground state of ${\mathrm{HD}}^{+}$ is seven orders of magnitude larger than the corresponding dipole polarizability. For the dipole polarizability of ${\mathrm{HD}}^{+}$ in the first excited state the high precision of the present method facilitates treatment of a near cancellation between two terms. For applications to laser spectroscopy of trapped ions we find tune-out and magic wavelengths for the ${\mathrm{HD}}^{+}$ ion in a laser field. In addition, we also calculate the first few leading terms for long-range interactions of a hydrogen molecular ion and a ground state H, He, or Li atom.

An extension of the nonadiabatic quantum molecular dynamics approach is presented to account for electron-nuclear correlations in the dynamics of atomic many-body systems. The method combines electron dynamics described within time-dependent density-functional or Hartree-Fock theory with trajectory-surface-hopping dynamics for the nuclei, allowing us to take into account explicitly a possible external laser field. As a case study, a model system of ${\mathrm{H}}^{+}+\mathrm{H}$ collisions is considered where full quantum-mechanical calculations are available for comparison. For this benchmark system the extended surface-hopping scheme exactly reproduces the full quantum results. Future applications are briefly outlined.

The role of electron-nuclear correlations, i.e., quantum effects in the nuclear motion in atomic collisions with complex targets, is discussed using the recently developed nonadiabatic quantum molecular dynamics with hopping method [Fischer, Handt, and Schmidt, paper I of this series, Phys. Rev. A 90, 012525 (2014)]. It is shown that the excitation process is nearly unaffected by electron-nuclear correlations as long as integral quantities are considered (total kinetic energy loss), whereas the relaxation mechanism of the molecular target is greatly affected (total fragmentation probability). To describe highly differential quantities (kinetic energy loss as a function of the scattering angle), however, the consideration of nuclear quantum effects during the initial excitation process is indispensable, even in collisions where one would expect purely classical behavior of the nuclei due to their small de Broglie wavelength. The calculations reproduce and explain in detail old but still unexplained experimental data of differential energy-loss spectroscopy in $\mathrm{He}+\mathrm{He}$ and $\mathrm{He}+{\mathrm{H}}_2$ collisions.

Photoinduced excitation and relaxation of organic molecules (${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{CH}}_2{\mathrm{NH}}_2^{+}$) are investigated by means of nonadiabatic quantum molecular dynamics with hopping (NA-QMD-H), developed recently [Fischer, Handt, and Schmidt, paper I of this series, Phys. Rev. A 90, 012525 (2014)]. This method is first applied to molecules assumed to be initially ad hoc excited to an electronic surface. Special attention is drawn to elaborate the role of electron-nuclear correlations, i.e., of quantum effects in the nuclear dynamics. It is found that they are essential for a realistic description of the long-time behavior of the electronic relaxation process, but only of minor importance to portray the short-time scenario of the nuclear dynamics. Migration of a hydrogen atom, however, is identified as a quantum effect in the nuclear motion. Results obtained with explicit inclusion of an fs-laser field are presented as well. It is shown that the laser-induced excitation process generally leads to qualitatively different gross features of the relaxation dynamics, as compared to the field-free case. Nevertheless, the nuclear wave packet contains all subtleties of the cis-trans isomerization mechanism as observed without a laser field.

We theoretically analyzed a scheme for optical trapping of neutral atoms by plasmonic nanotips. An analytic quasistatic model was developed for studying the requirements on geometry and material parameters needed for a successful trap generation. We then used the numerical solution of Maxwell's equations to study different types of tips and realized optical trapping potentials at distances of up to 90 nm away from the tip apex. A dipole approximation of numerical results is presented, allowing for capturing essential trap characteristics in a simple form. Finally, we analyze scattering rates, trap characteristics, and heating of an exemplary nanotip, revealing the importance of heat dissipation for the considered structures.

The ground state of a homogeneous Bose gas of hard spheres is treated in self-consistent mean-field theory. It is shown that this approach provides an accurate description of the ground state of a Bose-Einstein condensed gas for arbitrarily strong interactions. The results are in good agreement with Monte Carlo numerical calculations. Since all other mean-field approximations are valid only for very small gas parameters, the present self-consistent theory is a unique mean-field approach allowing for an accurate description of Bose systems at arbitrary values of the gas parameter.